Magnetic resonance imaging (MRI) is a medical imaging modality that can create pictures of the inside of a human body without using x-rays or other ionizing radiation. MRI uses a powerful magnet to create a strong, uniform, static magnetic field (i.e., the “main magnetic field”). When a human body, or part of a human body, is placed in the main magnetic field, the nuclear spins that are associated with the hydrogen nuclei in tissue water become polarized. This means that the magnetic moments that are associated with these spins become preferentially aligned along the direction of the main magnetic field, resulting in a small net tissue magnetization along that axis (the “z axis,” by convention). An MRI system also comprises components called gradient coils that produce smaller amplitude, spatially varying magnetic fields when a current is applied to them. Typically, gradient coils are designed to produce a magnetic field component that is aligned along the z axis, and that varies linearly in amplitude with position along one of the x, y or z axes. The effect of a gradient coil is to create a small ramp on the magnetic field strength, and concomitantly on the resonant frequency of the nuclear spins, along a single axis. Three gradient coils with orthogonal axes are used to “spatially encode” the MR signal by creating a signature resonance frequency at each location in the body. Radio frequency (RF) coils are used to create pulses of RF energy at or near the resonance frequency of the hydrogen nuclei. The RF coils are used to add energy to the nuclear spin system in a controlled fashion. As the nuclear spins then relax back to their rest energy state, they give up energy in the form of an RF signal. This signal is detected by the MRI system and is transformed into an image using a computer and known reconstruction algorithms.
The gradient coil assembly used in an MRI system may be a shielded gradient coil assembly that consists of inner and outer gradient coil assemblies bonded together with a material such as epoxy resin. Typically, the inner gradient coil assembly includes inner (or main) coils of X-, Y-, and Z-gradient coil pairs or sets and the outer gradient coil assembly includes the respective outer (or shielding) coils of the X-, Y- and Z-gradient coil pairs or sets. The Z-gradient coils are typically cylindrical with a conductor spirally wound around the cylindrical surface. The transverse X- and Y-gradient coils are commonly formed from a copper panel with an insulating backing layer. A conductor turn pattern (e.g., a fingerprint pattern) may be cut in the copper layer of the gradient coil.
During an MRI scan, acoustic noise and vibration can be generated by the MRI system. The acoustic noise and vibration can be uncomfortable and potentially harmful to both the patient and the scanner operator. There are several sources of acoustic noise in an MRI system including, for example, the gradient coils and the RF body coil. The acoustic noise may be generated by the vibration of the gradient coils when the coils are pulsed during imaging operation. Gradient coil vibration is created by forces applied to the gradient coil as a result of the interaction of the static magnetic field and the electrical currents in the gradient coil to create the desired field. While the net radial force on the gradient coil assembly can be zero, each half of the gradient coil cylinder can experience a large radial force. These radial forces result in vibration of each half of the gradient coil cylinder which can result in the excitement of some acoustic modes (e.g., fish mode, banana mode, etc.). In addition, the vibration of the gradient coil assembly can result in Helium boil off in the magnet cryostat.
It would be desirable to provide a system and apparatus for balancing the radial forces in a gradient coil to reduce vibration and acoustic noise.